Catalysts applied in the form of a coating to honeycomb bodies have been used for decades for the purification of exhaust gases from automobiles. The honeycomb bodies have parallel flow channels for the exhaust gases running through them. Ceramic honeycomb bodies are produced from highly refractory materials. The majority are made of cordierite, a magnesium-aluminium silicate. Further ceramic materials customarily used are silicon carbide, aluminium oxide, zirconium oxide, zirconium mullite, mullite, silicon nitride, barium titanate and titanium oxide. The honeycomb bodies are produced from these materials by extrusion and generally have an open pore structure.
The flow channels run through the honeycomb bodies from an entry end face to the exit end face. The channels generally have a square cross section and are arranged in a closely spaced grid over the cross section of the honeycomb bodies. The number of flow channels per unit cross-sectional area is referred to as the cell density and can be from 10 to 200 cm−2.
The catalytic coating of the honeycomb bodies is a dispersion coating which is applied to the honeycomb bodies using a usually aqueous suspension of the catalyst components. This coating will hereinafter also be referred to as a washcoat.
The catalyst components contain, for example, finely divided support materials having a high specific surface area to which the catalytically active components, usually the noble metals of the platinum group platinum, palladium, rhodium, iridium and ruthenium have been applied. The solids in the catalyst suspension are generally homogenized by wet milling before application to the honeycomb bodies. After milling, the solids of the suspension have an average particle size d50 in the range from 3 to 5 μm.
Examples of support materials are simple and mixed oxides, e.g. active aluminium oxide, zirconium oxide, tin oxide, cerium oxide or other rare earth oxides, silicon oxide, titanium oxide, and silicates such as aluminium silicate or titanates such as barium or aluminium titanate and zeolites. The various phases of active aluminium oxide of the transition series, which can be stabilized by doping with silicon oxide and lanthanum oxide or else by zirconium oxide and cerium oxide, have been found to be particularly useful as heat-resistant support materials.
If reference is made in the following to coating of the honeycomb bodies with a catalytically active layer, what is meant is the coating of the channel walls of the flow channels. Coating of the outer wall of the honeycomb body is undesirable. The amount of coating applied is generally based on the external volume of the honeycomb body in gram per litre. A person skilled in the art will be sufficiently familiar with customary methods of carrying out such coatings.
To meet the necessary requirements, the coating has to have a concentration which depends on the particular task. In general, the concentration has to be greater the more active and more ageing resistant the coating is to be. In practice, from 10 to 300 g/l are required, depending on the application. However, the maximum achievable concentration can for various reasons be below the catalytically required concentration. Thus, the adhesion of the coating decreases with increasing concentration and thus layer thickness. Furthermore, high layer thicknesses reduce the hydraulic diameter of the flow channels and thus increase the exhaust gas counterpressure (back pressure) of the catalyst.
There are applications, for example the oxidation of hydrocarbons and carbon monoxide in diesel exhaust gas (“diesel oxidation catalyst”), in which only a relatively low mass of catalyst in the range from 100 to 200 g per litre of honeycomb body volume is necessary for the reaction. A further increase in the mass of catalyst while maintaining the total noble metal content is in this case not associated with an activity advantage. In the case of other catalytic reactions, for example the storage and reduction of nitrogen oxides (“nitrogen oxide storage catalyst”) or the selective catalytic reduction of nitrogen oxides by means of ammonia (“SCR catalyst”), on the other hand, an increase in the mass of active material is desirable but is possible only within limits because of the abovementioned problems with the adhesion of the coating and the back pressure of the finished catalyst.
To reduce the high back pressure, U.S. Pat. No. 5,334,570 proposes relocating the catalytic coating into the pores of ceramic honeycomb bodies. The ceramic honeycomb bodies used in this patent had an open porosity of from 30 to 45% and an average pore diameter of from 3 to 10 μm. For this reason, catalyst materials which have colloidal particle diameters in the range from 0.001 to 0.1 μm, preferably from 0.001 to 0.05 μm, and penetrate into the pores of the honeycomb bodies when the honeycomb bodies are brought into contact with a corresponding colloidal coating dispersion were used for catalytic coating. According to the patent, the honeycomb bodies were dipped into the coating dispersion in order to contact them with the coating dispersion. In this way, from 90 to 95% of the colloidal washcoat particles could be deposited in the pores of the honeycomb bodies, so that the cross section of the flow channels was barely reduced by the coating and the back pressure was therefore increased only insignificantly.
Ceramic honeycomb bodies having a significantly increased porosity of from about 60 to 65% and average pore diameters of from 10 to 20 μm have been developed in recent years. The objective here was to make the channel walls permeable to the catalyst particles so that the particles can deposit not only as a layer on the channel surface but also in the pore system of the wall. It is therefore possible to achieve lower layer thicknesses for a comparable mass of catalyst or, conversely, higher loading concentrations at equal catalyst layer thickness [Tao et al., SAE 2004-01-1293].
To coat honeycomb bodies, the catalytically active, water-insoluble, pulverulent components are usually suspended in water or an organic liquid, milled and the substrate is subsequently coated by dipping into the suspension, by pouring the suspension over it or by sucking in or pumping in the suspension.
If the above-described, newly developed porous honeycomb bodies are used here, part of the catalytically active substances actually penetrates into the pore system of the honeycomb body and deposits there. However, the processes described do not allow the pores of the honeycomb body to be filled completely and thus be optimally utilized.